Help you easily realize a programmable LED driver~

With intelligent control systems, even the smallest details of their important collections can be brought to life through pre-set lighting scenes. These digital control systems integrate programmable LED drivers so that the LEDs can be activated as needed. Figure 1 shows an example of a 3-channel LED driver configuration.

Figure 1. Simplified schematic of an LED driver used to control three independent LEDs.

Each of the three output voltages of a digital-to-analog converter (DAC), in this case the AD5686 from Analog Devices, controls a voltage-to-current converter stage, with separate LEDs placed in the load path of each stage for each LED channel. All three converter stages are implemented with the ADA4500-2 operational amplifier (op amp) and a MOSFET connected to control the LED current. In theory, this LED current can be as high as several amperes, depending on the voltage source (VS) and the load resistance, which is 2Ω in this circuit. Therefore, choosing the right MOSFET is very important.

The quality of the DAC output voltage depends largely on the reference voltage VREF. A high-quality reference voltage should be used. The ADR4520 is such an example, as shown in Figure 1. It has very low noise, very high long-term accuracy, and excellent temperature stability.

Typical rail-to-rail amplifiers have some nonlinearity and crossover distortion due to the internal design of the ADA4500-2. Their input stage consists of two differential transistors in parallel: a PNP stage (Q1 and Q2) and an NPN stage (Q3 and Q4), as shown in Figure 2.

Figure 2. Simplified rail-to-rail bipolar transistor input stage in an op amp.

Depending on the common-mode voltage applied, the two input pairs generate different offset voltages and bias currents. If the common-mode voltage applied to the amplifier inputs differs by less than 0.7V from the positive or negative supply voltage (VS), only one of the two input stages will be activated. Then, only the errors (offset voltage and bias current) corresponding to the active stage will appear. If the voltage rises to 0.8V, both input stages will be activated. In this case, the offset voltage may change abruptly, resulting in so-called crossover distortion and nonlinearity.

In contrast, the ADA4500-2 has an integrated input-side charge pump that covers the rail-to-rail input range without the need for a second differential pair, thus avoiding crossover distortion. Other advantages of the ADA4500-2 include low offset, low bias current, and low noise components.

In this type of circuit, attention must be paid to the inductance created by the LED wiring in the load/current path. The wires are often several meters long and can cause undesirable oscillations if the correct compensation is not provided. Compensation in this circuit is achieved through a feedback path that returns the current measured by the shunt resistor to the input of the op amp. The existing resistor and capacitor circuit on the ADA4500-2 should be adjusted based on the resulting inductance.

Using the circuit shown in Figure 1, it is easier to implement a multi-channel LED driver that can be programmed by a DAC for precise lighting control applications. It is also important to make appropriate adjustments according to specific needs to avoid malfunctions.

The circuit described in this article shows a simpler way to create a programmable LED driver that is ideal for precise lighting control applications that require a compact, scalable, easy-to-power, and highly linear power supply. However, the size must be adapted to the requirements of the application to avoid any malfunctions due to the various inductances present, such as line inductance and parasitic inductance.